1. Field of the Invention
The invention relates generally to the field of telecommunications, and more particularly to mechanisms for providing outer loop power control in a wireless communication channel when data is intermittently transmitted over the channel.
2. Related Art
Wireless communication technologies are rapidly advancing, and wireless communication systems are utilized to provide a larger and larger portion of the communications capacity that is currently available to users. This is true despite the additional technological impediments that are faced in implementing a wireless communication system, as compared to a wireline system. For instance, wireless communication systems must deal with issues relating to power control between a base station and its mobile stations in order to maximize the performance of the system, whereas a wireline system does not.
One type of wireless communication system comprises a cellular CDMA (code division multiple access) system which is configured to support voice and data communications. This system may have multiple base stations which communicate via wireless channels with multiple mobile stations. (The base stations are also typically coupled via wireline networks to various other systems, such as a public switched telephone network.) Each base station communicates with a set of mobile stations that are within a sector corresponding to the base station. This base station is responsible for controlling power in communications between the base station and the mobile stations in order to minimize interference and maximize throughput, as well as enabling the mobile stations to conserve energy and thereby extend the amount of time during which they can be used.
Power control between the base station and a mobile station in this type of system is typically based upon an error rate associated with communications between the base station and the mobile station. The goal of power control is to control the transmitter's power such that transmitted data is decoded with a constant quality level. One measure of quality is the frame error rate, which is the fraction of transmitted data frames that are received in error. Ideally, the mobile station transmission power is adjusted to a level that results in the desired predetermined frame error rate. To do this, the power control typically has two loops: the inner-loop and the outer-loop. The inner-loop regularly measures the signal-to-noise ratio (SNR) at the base-station and compares it to a target SNR. This measurement may be performed on any channel or combination of channels that can be used as a power reference. For example, in cdma2000 this measure is typically done on the reverse link pilot channel (R-PICH). The result of the comparison is used to generate a power control command, which is relayed to the mobile station. For example, if the SNR measured at the base-station is below the target SNR the inner-loop will issue a command instructing the mobile station to increase its transmission power, and if the SNR measured at the base-station is above the target SNR the inner-loop will issue a command instructing the mobile station to decrease its transmission power. The outer-loop regularly updates the target SNR based on an estimate of the current decoding quality. For example, the outer-loop may increase the target SNR by 1 dB each time a frame is incorrectly decoded, and decrease the target SNR by 0.01 dB each time a frame is correctly decoded. In this manner, the target SNR for the inner-loop is adjusted to a level at which the predetermined, acceptable error rate is maintained.
While this type of power control algorithm is suitable for channels over which data is continually being transmitted, it is less well-suited to channels which are used intermittently. The problem in this situation is that, put very simply, there are periods during which there are no data frames that can serve as the basis for adjusting the target SNR. Although the inner-loop may operate on a signal that is continuously transmitted such as the R-PICH, the outer-loop does not have any signal to update the target SNR. In other words, when frames are being transmitted, errors in the frames can be identified, and the target SNR can be adjusted to achieve the desired error rate, but when no frames are being transmitted, there is no way to tell whether the target SNR should be adjusted upward or downward. Consequently, following a period during which no frames of data are transmitted, the target SNR level may not be set at the optimal level, and therefore the inner-loop may not be instructing the mobile station to transmit at the optimal power level. If the level is set too low, the frames that are initially transmitted are almost guaranteed to have errors. On the other hand, if the power level is too high, power is wasted and unnecessary interference is generated, potentially causing errors in the transmissions of other mobile stations. It would therefore be desirable to provide a mechanism through which a preferred target SNR level can be achieved in the absence of data transmissions.